hmm so if i repair a almost broken assault rifle into an AR with an Extended cap; brining it down to 0%... this actually means that it will deteriorate 2x quicker then if I used a brand new guy?
Didn't totally understood what you meant, but:
Deterioration = nothing when you upgrading your weapon.
Broken count = the only thing that affects speed of weapon deterioration.
So, it's better to you to sell almost broken <=99% deterioration items to vendor, than repair it and sell, because with broken count it'll lose it's worth.
And it's better DO NOT repair an item before upgrade it, you'll get 0% det item anyway, but if you'll repair it, it'll have on 1 broken counter more, so it'll deteriorate faster.
Total:
Use repair only if you have repair alt with uber repair skill who can critically repair an item without making it "seems like it seen few repairs" (I did it only once with 120% repair, tools in hands, and I repaired "looks little broken" 10mm pistol).
Use repair if this weapon isn't upgradable and >100% deteriotation and
you want to use it, if you want to sell it, better sell without repairing.